textbook

Though not that original, I want to recommend Timoshenko. Since many people have mentioned it already, I will discuss a Brief on Tensor Analysis by James Simmonds. Though not always useful, I sometimes use it to remember tensor rules that I have forgotten. The book is divided into chapters as follows:

I: Vectors and Tensors

II: General Bases and Tensor Notation

III: Newton's Law and Tensor Calculus

IV: Gradient, Del Operator, Covariant Differentiation, Divergence Theorem

Again, sometime it is not that useful and you spend your time trying to read it while not learning much, but it does come in handy sometimes. You can see the amazon link:

Amazon

For math problems, I sometimes consult Advanced Engineering Mathematics by Greenberg; however, it seems when I need it most it is not helpful. Sometimes, it seems no one is better than Wikipedia for obscure math topics.